Comparison of Evidence Toolmarks and Test Marks
A typical approach for the comparison of test marks versus marks on evidence items includes the following steps:
- Perform preliminary procedures according to laboratory protocols.
- Confirm that the evidence item bears class and microscopic marks of value for comparison purposes and that these marks are suitable for comparison. The use of a stereomicroscope at lower magnification is excellent for this purpose.
- Adjust the lighting of the comparison microscope from the rear to provide oblique or grazing illumination over the surface of the marks towards the examiner.
- Place the test marks on the right stage in a convenient orientation with the best marks possible in the center of the field of view and index this best area using a colored permanent marker.
- Place the evidence item on the left stage with the questioned marks in an appropriate orientation for comparison. In some circumstances, it may be necessary to adapt and improvise (depending on the nature, size, shape, weight, and bulk of the evidence) to ensure that the questioned area can fit into the space between the objective lens and the stage, while still maintaining the ability to shift it in the x, y, and z axes.
This can be accomplished in several ways, including the following:
- Remove the questioned marks from the evidence item. Before proceeding, obtain appropriate permission to modify the evidence.
- Use plasticene (a plastic paste) or clay as a foundation to secure the questioned area in an appropriate orientation.
- Fabricate a specimen mount.
- Cast the marks and compare these casts with casts of the test marks.
- Align the evidence marks with the test marks on either side of the optical hairline of the microscope to confirm the consistency of class characteristics.
- If the class characteristics are different, it will be possible to exclude the questioned tool.
- If the class characteristics are consistent, the examination should proceed.
- Manipulate the evidence item to search for individual characteristics similar to the best area of the test mark.
If an area of sufficient agreement is found, a corresponding index mark should be placed on the evidence using the same color permanent marker as was used to index the test mark.
- The area of best agreement should be documented, preferably by digital or conventional photography or other means, as determined by laboratory protocol. Images should be marked with the examiners initials, case identifier, degree of magnification, item numbers, and a description of the nature of the observed area.
If the evidence and test toolmarks cannot be matched with each other based on individual marks (no sufficient agreement), possible reasons for the inconclusive result include the following:
- The toolmarks were made by two different tools.
- Damage occurred that caused distortion, deformation, or elimination of microscopic detail in the toolmark.
- The tool was damaged, extensively used, abused, or remachined (i.e., sharpened) between the time the two marks were made.
- Numerous subclass characteristics may be present.
Potential remedies that could be used in certain circumstances to produce better microscopic detail in test marks include these:
- Remount both test and evidence marks end to end on their respective mounts and reexamine them.
- Smoke the evidence toolmarks with magnesium smoke to enhance detail. This technique is beyond the scope of this training and should be performed only by an experienced examiner.
- Prepare additional test marks in a different material, possibly a softer material or material more like the evidence item.
- Change the amount, direction, and angle of force employed to create the test marks.
|A study of crime scene documentation and investigative reports often provides insight into the precise orientation of a tool to a toolmarked area at the time the mark was made. This can save considerable time by reducing the number of possibilities.
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